Why don’t people buy stocks and sell them quickly when they go up?

So I'm new to stocks and I noticed that the shares I buy always go above the price I bought them for within the first few days. I was wondering why people don't just sell at this point and either buy the shares back in a few days when it goes down or just buy another stock. Why do alot of people hold for months/years when they could make a very good profit doing what I suggested?